US2224998A - Heat conditioned steel and method of conditioning - Google Patents

Description

Dec. 17, 1940. w. H. WOOD ETAL HEAT CONDITIONED STEEL AND METHOD OF CONDITIONING Filed March 23. 1937 7 a \T W I 5 Z I ,m a T w T /q m m m m o Patented Dec. 17, 1940 UNITED STATES HEAT CONDITIONED SI'EEL AND METEOD 0F CQNDITIQNING William H. Wood, South Euclid, and ()scar C.

Trautman, Pal-ma, (3

his

Application March 23, 1937,, Serial No. 13?,589

14 Claims.

Our invention relates to the metallurgy of carbon steels, and essentially to the control of constituents and structure in such a steel as same are modified by control of carbon trans formations caused therein by heat treatment. Particularly our objects are to secure wire, or similar material, of carbon steel having in exceptional degree those qualities which are desired when used in springs, and to provide a method whereby the formation of constituents and structure is so controlled, during heat treatment of commercial wire, and the like, of carbon steel, that the qualities'desired in springs are secured in a superior degree. To said ends we have discovered a method of controlled heat treatment whereby we secure in such an article of carbon steel ductility accompanied by hardness and strength in a degree greater than can be secured by any other method. Incidentally our method secures said desired physical characteristics without imparting to the wire undesirable physical qualities or defects, such as cracks, decarbonized areas or objectionable oxides on the surface. I

Our method may be applied in the treatment of steels containing alloying elements, which elements may however change the ranges of temperature at which particular carbon transformations will take place as is well understood in the art. But in the explanation of our method which follows we will consider same as used with commercial wire of steel, such as has a carbon content of from 0.50% to 370% and a manganese content of from 0.50% to 1.10%. It will be understood that our method is equally applicable for use in the heat treatment of wires, or similar articles, which are to be used for purposes other than use as springs, when such other purposes require qualities which may be secured by the practice of our method as herein indicated.

Our improved method has a high thermal efllciency and permits accurate and easy modifications of heating and quenching cycles, or unvarying maintenance. It is practiced with apparatus which is clean and flexible and easy to operate. In its heating step the article beingtreated is progressively brought rapidly, .and uniformly throughout a cross-section, to the desired temperature and instantly quenched, thereby avoiding any pre-quenching in an air gap. Other advantages will hereinafter appear.

To the accomplishment of the foregoing and related ends, said invention, then, consists of the means hereinafter fully described and particularly pointed out in the claims.

The annexed drawing and the following description set forth in detail one method and one product exemplifying our invention, such disclosed procedure and product constituting, however, but one of various applications of the primciple of our invention.

In said annexed drawing:

Fig. l is a photo-micrograph at 2000 diameters showing a cross-section of quench-hardened wire;

Fig. 2 is a photo-micrograph at 2000 diameters showing a oross-section of the same hardened wire after tempering or drawing;

Fig. 3 shows diagrammatically apparatus for the practice of our method; and

Fig. 4 is a time versus temperature curve for a particular wire as treated by our method.

In a steel which has been heated to a tom perature above its upper critical range, the iron will he in the gamma form, and carbon up to a 1.70% will be held in solid solution. This phase is called austenite.

If the steel cools slowly through. its critical. temperature there will be a precipitation of ferrite, which is alpha iron very low in carbon, from the solid solution if the carbon content is below the eutectoid composition, about .87% carbon, and cementite, FeaC. if it is above. This precipitation will continue as the temperature continues to drop until the carbon content of the remaining austenite reaches the eutectoid composition. When this point is reached, the austenite transforms, depositing ferrite and cementite simultaneously, because the iron changes to the alpha phase, and the solubility of carbon therein is low. The cementlte precipitated will assume the form of very thin plates which will lie in thin layers alternately with the ferrite, provided the rate of cooling is such that there is the time interval necessary for this arrangement of the products of the transforma-. tion. This structure so formed is known as pearlite, and in it the percentage of cementite and ferrite will be such that on analysis the structure will be found to contain 87% carbon, or the same percentage as was held in solid solution at the moment of transformation from gamma to alpha iron.

Hardness and strength in carbon steels being attributes of the free cementite dispersed homogeneously in the ferrite, it is necessary, when said attributes are desired, that the steel be quenched, from a temperature at which the iron is in the gamma form, so rapidly that the carbon released as cementite will be homogeneously dispersed throughout the ferrite, forming martensite, and that the above recited structural transformation into nearlite will be inhibited.

As disclosed in our co-pending application Serial No. 51,872, filed Nov. 27, 1935, we have discovered a new method by which wire and the like may be heated and instantly quenched as it attains a desired temperature. In said method we advance the material to be treated uniformly into a bath of electrically conductive liquid, through'which bath a heating current of electricity is supplied to, and caused to flow through, a section of such material defined between said bath and an electrode at a spaced distance therefrom, and in contact with said material. By regulating the distance between said electrode and said bath, or by modifying the current strength, or by changing speed at which the material is advanced, the material may be caused to enter the bath at a uniform desired temperature. The rate and degree of quenching resulting therefrom may be controlled by selective choice of a conductive medium in the quenching bath and by maintaining the said bath at a desired temperature. By our said method the percentages of pearlite or of martensite produced when the carbon transformation results from the quenching of a carbon steel wire may be very accurately controlled. By using an electrically conductive bath capable of absorbing heat at a rapid rate, and maintaining it at a comparatively low temperature, a very hard product is produced which is believed to be all martensite. By heating a wire by the method explained and maintaining the quenching contact bath at a temperature in the neighborhood of 1000 F. the formation of sorbite results, and if the wire is retained in said quenching contact bath for a number of seconds a form of steel will be secured such as heretofore has been secured by processes known as patenting. This process is claimed in our application No. 252,954 filed Jan. 26, 1939.

By controlled use of said method we have 45 produced a heat-treated wire of carbon steel which has novel characteristics which make it better adapted for use in springs than any wire of similar analysis of which we have knowledge. We have heated commercial steel wire by the 50 practice of our described method until progressively cross-sections thereof attained a temperature at which the steel became substantially austenitic, which is to say a temperature of the order of 1400 or 1600" F., depending upon the 55 analysis of the particular steel. These crosssections as they progressively attained their maximum heat were quenched in an electrical contact bath of molten low melting alloy maintained at about 425 F. and were then with-- 60 drawn for further quenching at a slower rate.

Referring to Fig. 3 which shows diagrammatically apparatus used in the practice of our method, a wire I of commercial steel is drawn through said apparatus by a take-up reel 2. A 65 section 3 of said wire is subjected to a heating current of electricity supplied thereto by the electrodes 4 and 5 as it is advanced through the apparatus. Current is supplied to said electrodes 4 and 5 from a suitable source 8. The advance 70 electrode 5 is molten metal, such as solder, in a bath '1, maintained at a desired temperature. The strength of current, length of section 3 and speed of wire travel are so adjusted that each cross-section of the wire I will attain a prede- 75 termined temperature as it entered the ba h 1 and is quenched. From the bath 1 the wire is drawn for further quenching which is preferably done in an oil bath 8.

The drastic quench from the temperatures of austenitic formation causes stresses within the steel which it has been found advisable to remove by prompt drawing. We have, therefore, reheated the wire to about 800 F. as it emerges from the oil quench, preferably by passing through a lead pot 9 maintained at the desired temperature.

Using commercial steel wire, we have by our described process produced spring wire having the following characteristics:

(a) a structure in which cemenrtite is predominantly dispersed homogeneously throughout ferrite in other than pearlite arrangement, and substantially without needle formations of martensite,

(b) high tensile strength,

(0) ductility with unusual hardness, and

(d) a superior surface finish free from objectionable oxides and de-carburization.

Using commercial steel wire of No. 11 gauge, .12 inch in diameter, containing .60 to .70% carbon and about 0.90% manganese we have heated it to a temperature of 1450 F. as it entered 'a quenching electrical contact bath of solder maintained at a temperature of 425 F. From this solder quench it was withdrawn when its temperature was reduced to substantially that of the bath and passed through an oil quench which reduced its temperature substantially to room temperature. The wire so treated had a characteristic martensitic structure as is seen in Fig. 1, which is a photo-micrograph of a crosssection of said steel at 2000 diameters. Said steel so treated had a very fine structure with an absence of needles of martensite, and a Rockwell hardness of 64 to 66 C. with an impact strength eight to ten times that secured in the same steel when hardened by conventional methods. To temper for spring purposes, this wire was reheated to 800 F. and quenched in soluble oil thereby reducing the hardness to 45 to 47 Rockwell C, with a structure shown in Fig. 2 which is a photo-micrograph of a cross-section at 2000 diameters, wherein it is noted that the homogeneously dispersed cementite has tended lto' form fine globules. In Fig. 4 a time versus temperature curve is shown covering the heat treatment given said wire. As will be understood from Fig. 4, the steel in the particular case graphed changed from the alpha to the gamma form of iron in about 12 seconds. The heating was carried slightly higher to insure complete transformation and at the end of about 13 seconds the wire was abruptly quenched to below 500 F. in less than a second. Three seconds thereafter the wire was approximately of the temperature of the bath 425 F. and was withdrawn and further quenched.'

Commercial steel wire which has been hardened by our described method, and tempered, will show a marked improvement in elasticity over what can be attained when similar wire is hardened by old methods of heat treatment. This may be readily demonstrated by torsion tests and by determination of elastic limits. Tests, using the Johnson 50% method of determination, have shown that uniformly our method of hardeningcommercial steel wire will result in an increase in elastic limit, in pounds per square inch, of more than 10% over that obtained when conventional methods of hardening are used.

It is our belief that these superior characteristics of wire treated by our method result because in the heating stage the wire is heated evenly throughout its cross-section and progressively throughout the length of the section being treated, whereby at any instant only a minute length has attained the maximum temperature and such minute length as heated to the desired maximum temperature is instantly quenched, thereby inhibiting the formation of undesired grain growth which occurs in steel held at a high temperature, and because in the quenching stage the temperature isdropped substantially instantly, from the maximum to which the steel under treatment is subjected that it may become austenitic, to a temperature below that at which the carbon rejected from the solid solution, because in excess of the minute percentage soluble in alpha iron, will form the structure known as pearlite. This temperature will vary as is well understood according to the analysis of the steel being treated. The time interval being too short for the formation of the pearlite arrangement, the rejected carbon will form free cementite (FesC) in substantially homogeneous dispersion. in alpha iron holding a trace of carbon in solution. Y

The speed of the heating is such that no part of the wire is at an oxidizing temperature for a time sufficient to permit any substantial oxidizing of the iron content and formation of scale, or any oxidizing of the carbon causing decarburization. Such oxidation may be completely inhibited by inclosing the portion of the wire adjacent the quenching bath with an envelope of gas which is inert insofar as the steel is concerned.

It is believed the speed of the quench is such that the transformation of carbon, that results upon conversion from gamma iron to alpha iron, is accomplished in a time interval too short to permit the cementite formed to assume the laminar relation with the ferrite, which is known as peariite. It will be understood that the formation of pearlite, or the laminate structure of cementite and ferrite, takes place only at a limited range of temperature, below the temperature at which the iron changes from the gamma to the alpha form, and that such formation in. such range of temperature requires able period of time. It is our belief that we inhibit the formation of any appreciable amount of such pearlite by our substantially instantaneous quenching of the austenite, or solid solution of carbon in gamma iron. Further, it is our belief that by limiting our rapid quench and arresting at a temperature below that of rapid pearlite formation from which it is more slowly cooled, we secure a homogeneous dispersion of cementite in ferrite, or martensite. This martensite formation having been secured, we relieve the stresses, and modify the martensite to secure a ductility desired, by an appropriate temper draw. It has been recognized that steel rapidly quenched from a temperature, at which it was austenitic, that is to say, at tained carbon in solid solution in gamma iron, to room temperatures would produce a steel which was martensitic in structure. Also such structure may be produced with a great decrease of internal stresses by a step-by-step quenching, or by very rapidly quenching to a temperature just below that at which pearlitic transformation will take place and then completing the quench at a which it held conslower rate. We have devised a method whereby such heating and step-by-step quenching may be applied to wire and the like in continuous operation with the necessary accuracy of control.

It will be understood that important improvements involved in our new method include (a) the heating of the wire by passage of an electric current, while continuously advancing the wire into a quenching bath, whereby the temperature is uniform in any particular cross-section of the wire, and progressively increases in the direction of wire advance; (b) the supply of heating current to the wire through the medium of the quenching bath, whereby the steel is quenched at the instant it progressively in cross-section attains the required temperature,

avoiding any pre-cooling before the quench and avoiding any holding at a temperature which will induce grain growth; and (c) the selection of an electrically conductive medium in the quenching bath whereby the degree of quench may be limited, and a desired rate of quench may be maintained.

Qther modes of applying the principle of our invention may be employed instead of the one explained, change being made as regards the product and method herein disclosed, provided the step or steps stated by any of the following claims or the equivalent of such stated step or steps be employed.

We therefore particularly point out and distinctly, claim as our invention:

1. The method of thermally conditioning carbon steels which comprises rapidly quenching the same from a temperature at which it has assumed the so-called austenitic form to a temperature which is below .the temperature range of pearlitic transformation and above the tem-- perature range of rapid martensitic transformation, and said quenching being in a metal bath having a rate of temperature reduction which will inhibit substantial pearlite transformation; and further quenching from such temperature at a rate of temperature reduction at which transformation is to a structure predominantly martensitic.

2. As an article of manufacture, steel having the composition and hardened by the method as defined in claim 1. I

3. In a method of heat treating steel, having the form of wire and the like, the steps of sucan appreci-.

cessively and progressively in continuous operation at uniformrate, raising cross-sections of the material being treated to a temperature at which the carbon of the steel is held in solid solution in gamma iron; quenching such sections in a bath of molten metal, as they reach their maximum temperature, to a temperature which is below that at which pearlite will form rapidly and above that of rapid martensite formation, which temperature when treating commercial steels will be in the neighborhood of 425 F.; and immediately quenching at a rate of cooling slower than the rate of the metal bath, whereby the structure of the steel becomes predominantly martensitic.

4. As an article of manufacture, 'steel having the composition and hardened by the method as defined in claim 3.

5. In the heat treatment of commercial steel, having the form of wire and the like, by cooling from a temperature above the upper critical temperatui'e to atmospheric temperature, the method which comprises heating the wire progressively in cross-sections to the desired temperature,

its resistance to the.

which will be above 1325 F. in a degree depending uponthe composition of the steel. by passing an E. M. F. therethrough between a contacting electrode and a bath of liquid metal while advancing such wire at a uniform speed into and through such bath; maintaining the metal bath at a temperature which will quench ti 1e wire to a temperature below the temperature zone in which pearlite forms in the particular steel, which bath temperature will be in the neighborhood of 425 1 7., and immediately further quenching to atmospheric temperature.

6. As an article of manufacture, steel having the composition and hardened by the method as defined in claim 5.

7. In a method of heat treating steel, having the form of wire and the like, the steps of advancing continuously the material to be treated longitudinally at uniform speed into an electrically conductive bath of molten metal; maintaining such bath at a temperature of approximately 425 F.; heating a section of the advancing material, defined at its forward end at the surface of such bat by paming a heating current of electricity through such bath and through and to such section of material; maintaining such current flow at a strength which will heat such material to constant desired temperature between 1350" F. and 1650 F. as it enters such bath; advancing such material from such bath when its temperature has been reduced to substantially that of the bath; further quenching at a cooling rate slower than the rate of such metal bath; and immediately temper drawing at approximately 800 F.

8. As an article of manufacture, steel having the composition and hardened by the method as defined in claim 7.

9. The method of heat-treating steel, having the form of wire and the like, which comprises heating same by its resistance to the passage of an electric current through a defined length thereof as it is advanced longitudinally at constant speed into a quenching bath of molten metal which defines the forward end of such defined length, maintaining such electric current at a strength which will heat the wire at the forward end of such defined length to an austenitic condition, and maintaining the molten metal bath at a temperature below the temperatures at lhlCh pearlite will be formed by carbon transformation.

10. As an article of manufacture, commercial carbon steel conditioned by the method of claim 9.

11. The method of thermally hardening steel by controlling the transformation of the carbon therein, which comprises quenching the same rapidly at the moment it reaches a maximum temperature above the critical temperature for such steel. such quenching being to a temperature below that at which pearlite will form in such steel and further quenching at a rate relatively slower.

12. As an article of manufacture, commercial carbon steel conditioned by the method of claim 11.

13. The method of thermally hardening steel by controlling the transformation of carbon therein which comprises heating the same, by its resistance to the passage of an electric current, to a temperature above its critical temperature; quenching, at its maximum temperamolten metal, to a temperature immediately below the temperature of substantial pearlite formations in such steel, whereby e. conversion which is predominantly martensitic is insured: and further in continuous operation quenching to atmospheric temperature at a relatively slower rate.

- 14. As an article of manufacture, a commer-- cial carbon steel conditioned by the method of claim 13.

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